The scale on a map is 10cm : 4km. If two cities are 80cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 10cm on the map corresponds to an actual distance of 4km. A distance of 80cm on the map is the same as 8 $\cdot$ 10cm. The actual distance between the two cities is 8 $\cdot$ 4km, or 32km.